In recent years, glass products used in the industry of optical communication, semiconductors, etc. are under very severe controls with respect to the purity of the constituting glass materials.
To produce such a highly pure glass product, there are mainly the following methods: (1) a method using sand-like natural quartz powder (what is called "sand") obtained by pulverizing natural quartz; to obtain more purified ones, (2) oxygen-hydrogen flame method, a method which comprises using a lump of fume, obtained by adhesion and growth on the substrate, of the fume generated by the hydrolysis of silicon tetrachloride in the oxygen-hydrogen flame; (3) a method whereby an alkoxysilane is hydrolyzed and gelled to obtain a silica gel powder, and then the silica gel powder is calcined to obtain a glass powder, which is then fused and shaped. However, among these methods, method (1) has limit in lowering extremely small amount of impurities content, method (2) has problem of expensive production cost.
Method (3), using silica gel, especially using silicon alkoxide as the starting material, quartz glass with low impurity content can be economically obtained compared to the method (2). However, by this method, silanol groups which the starting material has during the hydrolysis and gelation, will remain as residual silanol groups in the particles after the calcination, whereby, as compared with natural quartz powder, the product has a higher silanol content. If the silanol content is high, the viscosity at a high temperature tends to be low, and such a product is not useful for e.g. a crucible for drawing a silicon single crystal or for a diffusion furnace semiconductors (Japanese Unexamined Patent Publication No. 320232/1989). For such applications, it is said that the silanol content is preferably 100 to 50 ppm.
However, there is also such a problem that lowering residual silanol content to this level is technically difficult in industrial scale, and even when attained, long time calcination is necessary. If lowering of silanol content is not enough, there is also such a problem that the obtained shaped quartz glass product has small bubbles.
To lower the silanol concentration efficiently, there are also the following methods. One comprises calcining silica gel in a gas containing halogen dehydroxy agent such as chlorine(Japanese Unexamined Patent Publication No. 186232/1986). However, by this method, although silanol is efficiently reduced, the halogen atom remains, so the quality of the obtained shaped product cannot be satisfactory.
To reduce silanol content, there are also a method which comprises selecting a low water vapor partial pressure atmosphere for calcination (Japanese Unexamined Patent Publication No. 289416/1990), and a method in which calcination is carried out by introducing dry gas in the silica gel (Japanese Unexamined Patent Publication No. 83711/1992). By the former method, silanol is reduced by a long time calcination, but it can not necessarily shorten the calcination time. Namely, long time calcination is necessary. It is described that the latter method improves the silanol reducing rate, and it is explained that a precipitated silica gel having an average particle size of 120 .mu.m is used as a starting material, and a gas with low water vapor partial pressure is used, then the silanol content after the calcination can be reduced to no more than 60 ppm. By the former method, for example under high temperature of 1100 to 1250.degree. C., calcination is carried out by introducing a dry gas, and a synthetic quartz glass powder is obtained, then the obtained synthetic quartz glass powder is took out of the furnace and cooled. By such methods, the silanol content in he obtained product is from 60 to 100 ppm.
As described above, in the conventional methods, the attention was paid to the atmospheric moisture at high temperature, especially not less than 800.degree. C. The studies of the present inventors have clarified that, generally at a temperature not lower than 1000.degree. C., there is equilibrium between the silanol content in the silica solid and the water content in the atmosphere (moisture), and that the reversible desorption rate of silanol is fast. Therefore, such a method has been tried as to use dry air during calcination to reduce the silanol content in the synthetic quartz glass powder, as described above.
However, as the present inventors have made an intentional studying, they have found out that, to reduce the silanol content to the aimed value (not more than 50 ppm of silanol content to maintain the high temperature strength in high level and to reduce bubbling in the obtained shaped glass product), surprisingly, water absorbed reversibly upon cooling, namely silanol increase, cannot be neglected. This means that conventionally, more than necessary length of calcination was required, and silanol was reduced to an extreme degree, to compensate the increase of silanol during the cooling.
To lower water to be absorbed upon cooling, to cool quickly from high temperature can be considered, but in such case, quenching causes heat deterioration of the material. In a practical manner, cooling rate applicable to an industrial material (furnace material, crucible, etc.) needs for example at least several hours to cool from about 800 to 200.degree. C., so the increase of silanol cannot be restrained enough.
Next, the present inventors tried the latter method, that is, to introduce dry air in the silica gel powder during the calcination, and followed in an industrial scale (using a quartz container capable of treating several dozens of kilograms to several hundreds of kilograms at once.), but the effect described in the above described bulletin (Japanese Unexamined Patent Publication No. 83711/1992) was not necessarily obtained. The dry air used upon calcination has been considered generally that, the more the flux, the more it increases silanol reduction rate. So, in the above bulletin (Japanese Unexamined Patent Publication No. 83711/1992), it is described that "no less than 3 liter/Hr per 1 kg of silica to be calcined" is introduced, and that in the case when 10 liter/Hr is introduced (Example 2), more silanol content is reduced compared to the case when 2 liter/Hr is introduced (Comparative Example 3). But with the pulverized silica gel with average particle size of 150 to 300.mu.m, that the present inventors used, such a tendency could not be observed. And the present inventors have found out that, contrary to the conventional idea, calcining carried out under condition using dry gas flux in a specific range of extremely low level, silanol reducing rate increases. Also, they have clarified that, when flux is above this specific range, when the powder is filled up in high level in the container used for calcination, a phenomenon that the treated powder boils over from the container occurs, and the yield is extremely lowered.
Furthermore, it is clarified that, when calcination is carried out actually in an industrial size scale, there is a great uneveness of silanol content in the obtained synthetic quartz glass powder, depending to the place in the container during calcination. For example, it is found out that, filling up silica gel powder for about 80 percent in a heat resistant cylindrical container and calcining for a very long time by introducing dry air, the obtained synthetic quartz glass powder has a silanol content of from 50 ppm to 80 ppm, so there is even 30 ppm of difference according to the place in the container. Trying to lower silanol content evenly, by lengthning the calcination time, or by raising the calcining temperature, a great amount of cost is needed, and furthermore, sintering between particles occurs, and there is inconvenience such that after-process such as crushing after the calcination is needed.
As described above, any of the conventional technics has difficulty to the silanol reduction in an industrial scale, and a method for producing a synthetic quartz glass powder with evenly and low content of silanol was needed.